In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Coated cemented carbide inserts with binder phase enriched surface zone are used to a great extent for machining of steel and stainless materials. Through the use of a binder phase enriched surface zone an extension of the application area is obtained.
Methods of producing binder phase enriched surface zones on cemented carbides containing WC, cubic carbide phase and binder phase are known as gradient sintering and have been known for some time, e.g., through Tobioka (U.S. Pat. No. 4,277,283), Nemeth (U.S. Pat. No. 4,610,931) and Yohe (U.S. Pat. No. 4,548,786).
The patents by Tobioka, Nemeth and Yohe describe methods to accomplish binder phase enrichment by dissolution of the cubic carbide phase close to the insert surfaces. Their methods require that the cubic carbide phase contains some nitrogen, since dissolution of cubic carbide phase at the sintering temperature requires a partial pressure of nitrogen within the body being sintered exceeding the partial pressure of nitrogen within the sintering atmosphere. The nitrogen can be added through the powder and/or the furnace atmosphere during the sintering cycle. The dissolution of the cubic carbide phase results in small volumes that will be filled with binder phase, thus giving the desired binder phase enrichment. As a result, a surface zone generally about 25 μm thick consisting of essentially WC and binder phase is obtained. Although the cubic carbide phase is essentially a carbonitride phase, the material is herein referred to as a cemented carbide.
Cemented carbides with a binder phase enrichment formed by dissolution of the cubic carbide phase usually contain the cubic carbide forming elements tantalum, titanium and niobium. It has been disclosed in EP-A-1043416 that a positive effect on the machining properties can be obtained if the amount of niobium is kept below 0.1 wt %. Moreover, EP-A-0560212 and EP-A-0569696 disclose the use of hafnium and zirconium additions. The total as well as the relative amounts of these elements result in slightly different properties of the cemented carbide insert. Tantalum for example is known to inhibit grain growth of the tungsten carbide grains, and to be advantageous to the toughness behaviour of the insert. Niobium has been found to form a more pronounced binder phase depleted zone just beneath the binder enriched surface zone in gradient structured cemented carbides (Frykholm et al., Int. J. of Refractory Metals & Hard Materials, Volume 19 (2001) pages 527-538), which is likely to result in a more brittle behaviour. Tantalum gives a more even distribution of the binder phase in the zone enriched in cubic carbide phase.